1. Field of the Invention
This invention relates to a process for reducing the sulfur oxide content of a waste gas stream through the use of absorbents which can be reactivated for further absorption of sulfur oxides by contact with a hydrocarbon in the presence of a hydrocarbon cracking catalyst. More particularly, this invention relates to a method for reducing sulfur oxide emissions from the regenerator of a fluidized catalytic cracking unit.
2. Description of the Prior Art
A major industrial problem involves the development of efficient methods for reducing the concentration of air pollutants, such as sulfur oxides, in the waste gases which result from the processing and combustion of sulfur containing fuels. The discharge of these waste gas streams into the atmosphere is environmentally undesirable at the sulfur oxide concentrations which are frequently encountered in conventional operations. Such waste gas streams typically result, for example, from operations such as the combustion of sulfur containing fossil fuels for the generation of heat and power, the regeneration of catalysts employed in the refining of hydrocarbon feedstocks which contain organic sulfur compounds, and the operation of Claus-type sulfur recovery units.
Two fundamental approaches have been suggested for the removal of sulfur oxides from a waste gas. One approach involves scrubbing the waste gas with an inexpensive alkaline material, such as lime or limestone, which reacts chemically with the sulfur oxides to give a nonvolatile product which is discarded. Unfortunately, this approach requires a large and continual supply of the alkaline scrubbing material, and the resulting reaction products can create a solid waste disposal problem of substantial magnitude.
The second principal approach to the control of sulfur oxide emissions involves the use of sulfur oxide absorbents which can be regenerated either thermally or chemically. The process of the subject invention is representative of this second approach.
U.S. Pat. No. 4,001,375 to J. M. Longo discloses a process for removal of sulfur oxides from a gas which involves absorbing the sulfur oxides with cerium oxide followed by regeneration of the spent cerium oxide by reaction with hydrogen gas. This regeneration step results in the formation of a gas which contains a 1:1 ratio of hydrogen sulfide to sulfur dioxide and which may be fed directly to a Claus-type sulfur recovery unit for conversion into elemental sulfur. It is further disclosed that the cerium oxide may be supported on an inert support such as alumina, silica and magnesia. The patent does not, however, suggest that the spent cerium oxide could be regenerated by contact with a hydrocarbon in the presence of a hydrocarbon cracking catalyst. In addition, the patent fails to suggest that the nature of the support could be significant or that cerium can be combined with alumina and/or magnesia to effect an absorption of sulfur oxides which is enhanced as a consequence of synergism.
An article entitled "Selection of Metal Oxides for Removing SO.sub.2 from Flue Gas" by Lowell et al. in Ind. Eng. Chem. Process Des. Develop., Vol. 10, No. 3, 1971, is addressed to a theoretical evaluation of the possible use of various metal oxides to absorb sulfur dioxide from a flue gas. The authors evaluate 47 metal oxides from which they select a group of 16 potentially useful single oxide absorbents, which includes the oxides of aluminum, cerium and titanium. Magnesium oxide was eliminated from the group of potentially useful oxides because of an unfavorable sulfate decomposition temperature. This evaluation is based on the assumption that the absorbents would be regenerated thermally and does not consider the possibility of regeneration under reducing conditions. Consequently, there is no suggestion that any of these metal oxides could be regenerated by contact with a hydrocarbon in the presence of a hydrocarbon cracking catalyst.
The cyclic, fluidized, catalytic cracking of heavy petroleum fractions is one of the major refining operations involved in the conversion of crude petroleum oils to valuable products such as the fuels utilized in internal combustion engines. Such a process involves the cracking of a petroleum feedstock in a reaction zone through contact with fluidized solid particles of a cracking catalyst. Catalyst which is substantially deactivated by nonvolatile coke deposits is then separated from the reaction zone effluent and stripped of volatile deposits in a stripping zone. The stripped catalyst particles are separated from the stripping zone effluent, regenerated in a regeneration zone by combustion of the coke with an oxygen containing gas, and the regenerated catalyst particles are returned to the reaction zone. In the application of this process to sulfur-containing feedstocks, catalyst is deactivated through the formation of sulfur-containing deposits of coke. In conventional processes, the combustion of this sulfur-containing coke results in the release of substantial amounts of sulfur oxides to the atmosphere. U.S. Pat. No. 3,835,031, to R. J. Bertolacini et al. discloses a method for the reduction of these sulfur oxide emissions through the use of a cracking catalyst comprising a zeolite in a silica-alumina matrix which has from about 0.25 to about 5.0 weight percent of a Group IIA metal or mixture of Group IIA metals distributed over the surface of the matrix and present as an oxide or oxides. The metal oxide or oxides react with sulfur oxides in the regeneration zone to form nonvolatile inorganic sulfur compounds. These nonvolatile inorganic sulfur compounds are then converted to the metal oxide or oxides and hydrogen sulfide upon exposure to hydrocarbons and steam in the reaction and stripping zones of the process unit. The resulting hydrogen sulfide is disposed of in equipment conventionally associated with a fluid catalytic cracking unit. Similarly, Belgian Pat. No. 849,637 also is directed to a process wherein a Group IIA metal or metals is circulated through a cyclic fluidized catalytic cracking process in order to reduce the sulfur oxide emissions resulting from regeneration of deactivated catalyst. The disclosures of these patents do not, however, suggest the desirability of combining a rare earth metal with the oxide of a Group IIA metal such as magnesium oxide or calcium oxide.
Belgian Pat. No. 849,636 and its counterpart, U.S. patent application Ser. No. 748,556, disclose a process similar to that set forth in U.S. Pat. No. 3,835,031, which involves the removal of sulfur oxides from the regeneration zone flue gas of a cyclic, fluidized, catalytic cracking unit through the use of a zeolite-type cracking catalyst in combination with a regenerable metallic reactant which absorbs sulfur oxides in the regeneration zone and releases the absorbed sulfur oxides as hydrogen sulfide in the reaction and stripping zones of the process unit. It is taught that a suitable metallic reactant comprises one or more members selected from the group consisting of sodium, scandium, titanium, chromium, molybdenum, manganese, cobalt, nickel, antimony, copper, zinc, cadmium, the rare earth metals and lead, in free or combined form. In addition, it is disclosed that the metallic reactant may be supported by an amorphous cracking catalyst or a solid which is substantially inert to the cracking reaction. Silica, alumina and mixtures of silica and alumina are mentioned as suitable supports. There is no specific teaching, however, of the desirability of combining any particular rare earth metals with inorganic oxides selected from the group consisting of the oxides of aluminum, magnesium, zinc, titanium and calcium. The disclosure contains no suggestion that such a combination would afford a synergistically enhanced reduction of regenerator sulfur oxide emissions.
Belgian Pat. No. 849,635 and its counterpart, U.S. patent application Ser. No. 748,555 are also directed to a process of the type set forth in U.S. Pat. No. 3,835,031 and Belgian Pat. No. 849,636, and teaches that an improved reduction of regeneration zone sulfur oxide emissions can be achieved by combining a sulfur oxide absorbent with a metallic promoter. The metallic promoter comprises at least one free or combined element selected from the group consisting of ruthenium, rhodium, palladium, osmium, iridium, platinum, vanadium, tungsten, uranium, zirconium, rhenium and silver. The sulfur oxide absorbent comprises at least one free or combined element which is selected from the group consisting of sodium, magnesium, calcium, strontium, barium, scandium, titanium, chromium, molybdenum, manganese, cobalt, nickel, antimony, copper, zinc, cadmium, lead and the rare earth metals. Although the metallic promoter enhances the ability of the absorbent to absorb sulfur oxides in the regeneration zone of a cyclic, fluidized, catalytic cracking unit, the more active promoters such as platinum and palladium also promote the formation of nitrogen oxides and the combustion of carbon monoxide in the regeneration zone. Since the discharge of nitrogen oxides into the atmosphere is environmentally undesirable, the use of these promoters has the effect of substituting one form of undesirable emission for another. The ability of these promoters to enhance the combustion of carbon monoxide in the regenerator is also undesirable in those situations wherein the regenerator vessel and associated equipment, such as cyclones and flue gas lines, are constructed of metals such as carbon steel which may not be able to tolerate the increased regeneration temperatures which can result from enhanced carbon monoxide combustion.
U.S. Pat. No. 4,146,463 to H. D. Radford et al. discloses a process wherein a waste gas containing sulfur oxides and/or carbon monoxide is conveyed to the regeneration zone of a cyclic, fluidized, catalytic cracking unit wherein it is contacted with a metal oxide which reacts with the sulfur oxides to form nonvolatile inorganic sulfur compounds. This patent teaches that suitable metal oxides include those selected from the group consisting of the oxides of sodium, the Group IIA metals, scandium, titanium, chromium, iron, molybdenum, manganese, cobalt, nickel, antimony, copper, zinc, cadmium, lead and the rare earth metals. In addition, the patent teaches that the metal oxide may be incorporated into or deposited onto a suitable support such as silica, alumina and mixtures of silica and alumina. The teaching of this patent fails to suggest the combination of specific rare earth metals with one or more inorganic oxides selected from the group consisting of the oxides of aluminum, magnesium, zinc, titanium and calcium. In addition, there is no suggestion that such a combination could produce improved results as a consequence of synergism.
U.S. Pat. No. 4,071,436 to W. A. Blanton et al. teaches that alumina and/or magnesia can be used to absorb sulfur oxides from a gas and the absorbed sulfur oxides can be removed by treatment with a hydrocarbon. It is further disclosed that sulfur oxide emissions from the regenerator of a cyclic, fluidized, catalytic cracking unit can be reduced by combining alumina and/or magnesia with the hydrocarbon cracking catalyst. Similarly, U.S. Pat. Nos. 4,115,249 (W. A. Blanton et al.), 4,115,250 (R. L. Flanders et al.) and 4,115,251 (R. L. Flanders et al.) teach the utility of alumina or aluminum to absorb sulfur oxides in the regenerator of a cyclic, fluidized, catalytic cracking unit. The disclosures of these patents do not, however, mention the rare earth metals or suggest that the combination of specific rare earth metals with alumina and/or magnesia could give improved results.
U.S. Pat. No. 3,899,444 to R. E. Stephens is directed to the preparation of a catalyst support which consists of an inert substrate or core which is coated with an alumina containing from about 1 to about 45 weight percent, based on the alumina, of a rare earth metal oxide which is uniformly distributed throughout the alumina coating. It is disclosed that the inert substrate may include such refractory materials as zirconia, zinc oxide, alumina-magnesia, calcium aluminate, synthetic and natural zeolites among many others. Similarly, U.S. Pat. No. 4,062,810 to W. Vogt et al. discloses compositions comprising cerium oxide on an aluminum oxide support.
U.S. Pat. No. 3,823,092 to E. M. Gladrow describes the treatment of a zeolite-type hydrocarbon cracking catalyst with a dilute solution containing cerium cations or a mixture of rare earth cations having a substantial amount of cerium in order to improve the regeneration rate of the catalyst. The resulting catalyst contains between about 0.5 and 4.0 percent of cerium oxide and it is further disclosed that the catalyst matrix may contain from 5 to 30% alumina. Similarly, U.S. Pat. No. 3,930,987 to H. S. Grand describes a hydrocarbon cracking catalyst comprising a composite of a crystalline aluminosilicate carrying rare earth metal cations dispersed in an inorganic oxide matrix wherein at least 50 weight percent of the inorganic oxide is silica and/or alumina, and the rare earth metal content of the matrix is from 1 to 6 percent expressed as RE.sub.2 O.sub.3. Also, U.S. Pat. No. 4,137,151 to S. M. Csicsery discloses a composition comprising lanthanum or a lanthanum compound in association with a porous inorganic oxide which may be the matrix of a zeolite-type cracking catalyst. These patents contain no mention of sulfur oxides and fail to suggest that the combination of specific rare earth metals with specific metal oxides, such as alumina, could afford an improved sulfur oxide absorbent which can be regenerated by contact with a hydrocarbon in the presence of a hydrocarbon cracking catalyst.
Alumina is a component of many different catalyst compositions which have been developed for use in the cracking of hydrocarbons. A synthetically prepared amorphous cracking catalyst, which received wide commerical use shortly after the development of fluidized bed cracking techniques, contained about 13% alumina and 87% silica. Subsequently, amorphous silica-alumina catalysts were developed and used commercially which contained about 25 to 30% alumina. In addition, silica-magnesia catalysts were also developed and used commercially. These silica-magnesia catalysts contained about 20% magnesia in addition to about 15% alumina and about 65% silica. At the present time, most if not all commercial cracking catalysts contain a crystalline aluminosilicate or zeolite which is distributed throughout an amorphous silica-alumina matrix.